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SQUID-NMR study of methyl quantum tunneling in a series of carboxylic acids

✍ Scribed by B. Black; G. Majer; A. Pines

Publisher
Elsevier Science
Year
1993
Tongue
English
Weight
401 KB
Volume
201
Category
Article
ISSN
0009-2614

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✦ Synopsis

Quantum tunneling frequencies of methyl groups at low temperature can be measured directly by low-field nuclear magnetic resonance (NMR) using our d&QUID (superconducting quantum interference device) spectrometer. From NMR spectra recorded in magnetic fields below 20 mT, tunneling frequencies between I40 and 250 kHz were measured for a homologous series of carboxylic acids at 4.2 K. For solids with equivalent crystal structures, we observe an exponential correlation between the tunneling frequency and the enthalpy of fusion; from this correlation we obtain information about the contribution of intermolecular interactions to the energy barrier for methyl rotation.

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N M R studies were conducted on the a-COOH groups of the amino acids Val, Leu, Ile, His and Pro and on the y-COOH group of Glu. Limit chemical shifts, downfield from external water, were 249-257 ppm in aqueous solution at low pH, and 234-243 ppm in HS0,F. The relative nuclear screening order for a-C